I am using DDD for 3 of my applications that use the same domain layer (domain services) via 3 different application (workflow) services. Infrastructure & UI layer don't have access of domain services. I'm also passing all external dependencies like repository via domain service constructor. So, do I need to create interface on domain services? Note that I have defined interface for application services.
Why are you looking for a reason? You can always extract an interface once you see a need for it.